Frequency independent split beam antenna



March 1951 R. H. DU HAMEL ETAL 2,977,597

FREQUENCY INDEPENDENT SPLIT BEAM ANTENNA Filed April 6, 1959 2 Sheets-Sheet 1 &

ION

B ing-11 \NVENTORS Q Ray/worm) H. DUHAMEL j FRED R. ORE

ATTOR N ENS March 28 1961 R. H. DU HAMEL EIAL 2,977,597

FREQUENCY INDEPENDENT SPLIT BEAM ANTENNA Filed April 6, 1959 2 Sheets-Shet 2 Ili G 5 \NVENTORS RqyMolvo H. DuHAMEL FRED R. ORE

ATTOR N Eb S United States Patent FREQUENCY INDEPENDENT SPLIT BEAM ANTENNA Raymond H. Du Hamel and Fred R. Ore, Cedar Rapids, Iowa, assignors to Collins Radio Company, Cedar I Rapids, Iowa, a corporation of Iowa Filed Apr. 6, 1959, Ser. No. 804,356

11 Claims. (Cl. 343-908) resemblance to the present invention, a basic structural difference exists which is responsible for a strikingly different radiation pattern. The embodiments of the prior application have a non-image structural relationship between opposite sides of the antennas to obtain a welldefined single unidirectional radaition lobe off of the apex end of the antenna. On the other hand, the present invention provides an antenna structure in which opposite sides have an image relationship and obtains two welldefined radiation lobes off the apex end of the antenna; wherein the two lobes have maximum values much less than 180 apart with a very deep" and well-defined pull in their H-plane pattern. "The parameters, a, T and \l/ as defined in the prior application are also applicable to the present invention.

It has been found with the present invention that both the input impedance and the radiation pattern are substantially frequency independentover and indefinitely wide range, in spite of great deviation from the complementary principle found in antenna theory. The present invention is even farther removed from the cornplementary principle than the antenna in the previouslycited application. That is, in Figure 1 of the prior ap plication if r11 is allowed to approach 180, if the teeth are circularly contoured about the apex, and if a-I-[i is allowed toapproach 180, the complementary principle is satisfied. However, in the present case, even if the same hypothetical changes are made, the complementary principle still cannot be satisfied; because the image relationship yet exists between opposite sides. Nevertheless, we have discovered conditions under which frequency independence can be obtained with the image relationship existing. However, we have found that less variation in the parameters t, or and 1- is permissible than in the case of the prior non-image logarithmically periodic antenna. It is therefore the principal object of this invention to provide an antenna which obtains a well-defined split-beam radiation pattern in its H-plane that remains substantially unchanged over an indefinitely large range of operating frequencies.

It is another object of this invention to provide an antenna which maintains a very nearly constant input impedance over an indefinitely large frequency range.

. It is still another object of this invention to provide an antenna in which a ground-plane can be placed through its center to eliminate the need for its opposite image structure.

It is a further object of this invention to provide an antenna which can be positioned over ground to provide a single frequency-independent high-angle horizontallypolarized radiation lobe for point-to-point high-frequency communication.

Further objects, features, and advantages of this invention will become apparent to one skilled in the art upon further study of the specification and the accompanying drawings in which:

Figure 1 illustrates a back-view of one form of the invention;

Figure 2 shows a top-view of the antenna in Figure 1;

Figure 3 provides a three-dimensional free-space view of the invention-and its accompanying radiation pattern.

Figure 4 illustrates an elevational view of another form of the invention positioned over a ground-plane; and

Figure 5 shows a top-view of the antenna in Figure 4.

The form of the invention shown in Figures 1, 2, and 3 is first considered. It is made from rod or wire members formed into the illustrated structure. An apex .12 of the antenna, found at its pointed end, provides a reference point for its angular structure. The antenna includes half-portions 1t and 11 which are generally triangular in shape and have adjacent vertices which tend to meet at point 12; although they never quite meet since the antenna terminals are provided by these ver-tices. Each half-portion 10 or 11 encompasses an angle on and is bisected by a metallic rod 13 or 14 respectively passing down the center of each half-portion and connecting electrically to each transverse rod or wire.

The half-portions 1t) and 11 are oriented in separate planes that intersect along the Y-axis in Figure 3, which passes through apex point 12. The angle between the planes is defined as 0. Angle 30 may be varied and is one of the parameters of the system.

The half-portions 10 and 11 in Figure l are identically constructed and are positioned as images of each other, if a mirror plane is considered to be positioned to' bisect angle #1. a

Each half-portion 10 or 11 in Figures 1 and 2 is formed into trapezoidal teeth. The teeth 10A, 10B 10N alternate on opposite sides of their center-rod 14 in a direction to the apex. Likewise, teeth 11A, 11B 11N alternate on the other half portion 11.

Each half-portion 10 and 11 is defined in the same by parallel sides such as 21 and 22 or 22 and 23, formed of transverse rods, with the longer side of each tooth be ing more distant from apex point 12. Each tooth is bounded on its outer side by the lines defining the angle a and is bounded on its inner side by center rod 14 or 13. Any two continuous and opposite teeth, such as 10A and 10B, are considered a period of structure. Hence, three transverse rods are included in any single period of structure and between any two adjacent periods there will be a transverse rod in common. The parameter 1' determines the structural periods of the teeth and' is deof the same structural periods. Withinany single period 3 of structure, the relationship between the distances from the apex of the parallel sides of any tooth is as follows:

where denominator is greater, and is constant for a given antenna design. For .logarithmic symmetry within each period of structure, a=

There is no theoretical limit as to the overall length R of the periodic structure. Thus, the structure may continue until the largest transverse rod 21 is as long as desired, and its length controls the lowest frequency of operation for the antenna. In general, its low-frequency limit will be that frequency at which its length is about half-wave length.

At the other extreme near the apex, the teeth become smaller and smaller. As a practical matter, the teeth cannot become infinitesimally small. Thus, the periodic structure must stop at some minimum-sized tooth, and the remainder of the structure to the apex is non-periodic. The length of the minimum sized tooth determines the highest frequency of the antenna range. Hence, the teeth should be continued toward the apex until the smallest tooth stops less than one-quarter wavelength at the highest operating frequency of the antenna; and it is preferable that it be about 0.1 wave-length at the highest frequency. The reason for this is to prevent the non-periodic structure near the apex from becoming a resonant dipole within the operating frequency range. That is, if the nonperiodic structure should radiate, it would provide a bentdipole pattern which has no relationship to the split beam radiation pattern of the periodic structure, a wrong polarization which would be transverse to the polarization of the periodic structure, and an input-impedance characteristic which would vary with frequency and would differ from the input impedance provided by the periodic structure.

However, the shape of the non-periodic portion of the antenna structure near its apex is not important, and it therefore may be triangular or it may be nothing more than an extension of the center rod toward the apex.

Center rods 13 and 14 have their ends as close together at the apex as possible while maintaining them electrically separated, so that opposite sides of an incoming transmission line can be connected to them to transfer energy to or from the antenna.

Although the best results are obtained when the diameter of the rod or wire of the structure varies linearly with distance from the apex, little deterioration in performance is observed by making them all from rod or wire having the same diameter. Thus, it has been found in practice that rod or wire of the same diameter may be used for the entire structure without any significant deterioration of performance for operating frequency bandwidths of the order of to 1. Great structural simplification results from being able to use the same diameter rod or wire throughout the structure.

Although a trapezoidal-toothed structure is shown in this application, the other types of logarithmicallyeri odic tooth-shapes given in the previously cited patent application, such as triangularly-shaped teeth, may be used. However, an advantage of trapezoidal over triangularly shaped teeth is that the trapezoidal type has been found to allow the antenna structure to be about 20% smaller in overall size for a given low-frequency limit, which can be significantly advantageous in practice.

The input impedance of the antenna is about 150 ohms. Since the antenna provides a balanced voltage at its apex, a balanced line may have opposite sides connected to the vertices of opposite half-portions; and the line'may be brought to the vertices in any manner which does not interfere with the radiation pattern. Thus, the line may be supported and brought forward as a bisector of angle e, or it may be brought away from the apex in a direction transverse to the E-plane of the antenna until sufficiently distant from it to not affect its pattern; and then it may be supported in any direction. A sufficient distance is a few wavelengths at the lowest operating frequency. For unbalanced transmission lines, which are commonly of the coaxial type, a balanced-to-unbalanced transformation is obtained as shown in Figure 3 by substituting an unbalanced line 31 for center rod 14 and soldering its outer conductor to the transverse rods in place of the center rod. The inner conductor of the line is extended across the small gap at the apex to the opposite halfportion vertex to which it is connected.

Figure 3 illustrates three-dimensionally the antenna and the type of split-beam radiation pattern obtained from it. It is noted that there are two radiation lobes 51 and 52 which extend away from the apex end of the antenna. These lobes are each elliptical in cross-section and have maximum intensity radials 43 and 44 which are separated by an angle 6 which is much less than A particular model of the invention had the following parameters: oc=14.5, -r=0.85, \//=29, and a length of each center rod of 60 centimeters to provide a low frequency limit of 1000 me. The maximum E-plane beamwidth at half power points for each lobe was about 60, the H-plane beam-width of each lobe was about 60, and the front-to-back ratio was about 18 decibels. The angle 6 between the two lobes was about 70 in the H-plane. The resulting gain of this antenna was slightly better than 10 decibels over a dipole, the radiation pattern was substantially frequency independent over a l0-to-1 bandwidth, and the input impedance variation maintained a standing wave ratio of less than 1.2 to 1.

It has been found that substantial frequency independence for the input impedance of this invention requires that angle l/ be equal to or greater than angle a.

It has been found that because of the image relationship between the two half-portions 10 and 11 of this invention, it is possible to dispense with one half-portion by providing a ground-plane at an angle of 02 to the single used half-portion. Thus, for large antennas operating down to the low end of the high-frequency range, the earth itself may be used as a ground-plane where it has sufiicient conductivity.

However, with the embodiment of Figures 4 and 5, a horizontally polarized single beam is provided, which represents one of the two beams of the split-beam embodiment. The second beam cannot be maintained in Figure 4 because it would go into the earth. Instead, there is direct and reflected energy to provide the single beam 41, which points upwardly at the vertical angle 5/2.

Furthermore, the embodiment of Figures 4 and 5 has an important advantage over the prior-described embodiments of the invention. To illustrate this advantage, it must be realized that the frequency-independent radiation pattern of the prior embodiments is obtained in free-space. Where the prior embodiments of Figures 1, 2 and 3 are mounted within a few wave-lengths above a conducting surface such as ground, reflections cause a change with frequency of the vertical radiation pattern of the antenna, although its horizontal pattern remains frequency independent. Thus the vertical pattern will break up into lobes which vary in number, angle, and size as a function of frequency.

However, with the ground-plane system shown in Figures 4 and 5, the vertical pattern as well as the horizontal pattern is maintained frequency independent. This occurs because half-portion 81 is oriented with respect to ground so that it and its image form an antenna similar to Figure 3, which is basically frequency-independent.

Nevertheless, when the antenna of Figures 1, 2 and 3 is positioned to provide vertically-polarized waves as shown in Figure 3, its split-beam pattern maintains azimuthal frequency-independence which is needed for direction-finding purposes, with breakup of the vertical pattern causing little if any difficulty.

Hence, the embodiment of Figure 4 is extremely useful for certain types of long-distance high-frequency communications. In long distance radio communications, it is necessary to vary the operating frequency over a frequency range of 4- or 5-to-1 during a year. For pointto-point communication, it is of paramount importance that the vertical pattern of the antenna be undistrubed during frequency change. No prior antenna is known which is capable of this characteristic.

In Figures 4 and 5, posts 51 and 52 support the back end of the antenna and an insulated stake 54 to support its front end. Conducting wire is woven back and forth to provide the trapezoidal teeth of a half-portion similar to either found in Figures 1, 2 M3, except that suspension-type support is provided in Figures 4 and ,5. The suspension supports are provided between post 54 and posts 51 and 52 by bridging between the ends of the teeth with single insulators 63 where the gap is small and with a pair of insulators 61 joined by a wire 62 where the gap is large. The wires 62 are insulated from the antenna and do not interfere with its pattern.

The suspension supports are drawn taut at the posts at insulators 81 to minimize the catenary shape of the antenna caused by the suspension structure. The catenary structural distortion of angle a and 11/ 2 has not been found to interfere significantly with the frequency independence of the antenna for bandwidths less than -tol. The angle a for the catenary maybe considered the angle defined by posts 51, 54 and 52. However, its H- plane beamwidth has been found to be slightly greater than that of the non-catenary antenna for equal angles a as defined above. A slight catenary with respect to angle ill/2 does not appear to have any significant effect upon the vertical pattern. The connection height of the antenna to posts 51 and 52 can be made adjustable to allow variation in angle 0/2 which permits control of the vertical angle 6/2 of the beam. As insulators 81 are raised higher to. make W2 greater, the vertical angle 8/2 is made smaller to angle the beam closer to the horizon. Likewise in the embodiments of Figures 1, 2 and 3, as angle W2 is made larger, angle 6 is made smaller, and vica versa. Angle 0/2 should not be made less than about oc/Z since the input standing wave ratio becomes undesirably large when the half-portion is angled close to ground.

In many situtations, the ground will have insufficient Although this invention has been described with re spect to particular embodiments thereof, it is not to be so limited as changes and modifications may be made therein which are within the full intended scope of the invention as defined by the appended claims.

We claim:

1. A split-beam logarithmically periodic antenna comprising two half-portions, each half portion being generally triangular in shape and having apexes positioned adjacent to each other but electrically separated, and angle on defining the lateral boundaries of each half-portion, with the apex of the angle being the apex of a respective h-alflportion, a central-conducting member included along each half-portion from its apex, a plurality of conducting teeth formed in each half-portion, with said teeth extending laterally from the central conducting member, said teeth having inner and outer sides, with said teeth being positioned alternately on opposite sides of said centralconducting member, the radial distances from the apex of the inner sides of adjacent teeth on one side of said central-conducting member having a geometric sequence ratio of 1, the ratio of the radial distances from the apex of inner to outer sides of a given tooth being a', transmission line means having opposite sides connected to the respective apexes, and the half-portions positioned with one being the image of the other with respect to an imaginary mirror plane located midway between the two half-portions.

2. A periodic antenna as defined in claim 1 in which p is the angle between the two half-portions, and 11/, is approximately 20:.

3. A periodic antenna as defined in claim 1 wherein said teeth are trapezoidal in their contours.

4. A periodic antenna as defined in claim 1 in which said teeth are formed of rod-like conducting members positioned about the contour of said teeth.

5. A periodic antenna as defined in claim 4 in which the central conducting member is a rod-like conductor positioned down the center of each respective half-portion and electrically connected to each of the rod-like conducting members forming the teeth.

6. A logarithmic periodic antenna as defined in claim 4 in which the lines bounding angle a are distorted slightly into a catenary shape.

7. A horizontally polarized antenna having both vertical and horizontal radiation patterns substantially inconductivity to provide a required amount of reflection.

12, to the back-end of the antena, and should be at least one wave-length wide at the lowest frequency of operation and be symmetrically located with respect to center wire 71. The groundplane may be constructed of wires parallel to the transverse wires of the antenna.

The ground-plane wires may be most economically positioned by spacing them with the same spacing as the transverse wires above in the antenna. However, in the ground-plane in front of the apex, the wires should be spaced apart by approximately the width of the smallest'tooth in the antenna. Acoaxial cable may be brought on or under the ground from any direction to the apex, where its inner conductor is connected to the end of center wire 71 at the apex. The input impedance of the antenna in Figures 4 and 5 is 75 ohms which is onehalf that of the embodiment of Figures 1, 2 and 3. Therefore, a coaxial cable having a characteristic impedance of 75 ohms provides a direct match.

The half-portions described herein may have solid teeth, straight or curved, as given in the prior cited application.

dependent of frequency, comprising an antenna portion being generally triangular in shape and having an apex angle a, a ground-plane positioned below said antenna portion and forming an angle W2, the apex of said antenna portion being close to the ground-plane with electrical separation, a central-conducting member included along the length of said antenna portion, a plurality of conducting-teeth formed in said portion with said teeth extending laterally from said central-conducting member, with said teeth being alternately arranged on opposite sides of said central-conducting member, the radial distances from the apex of inner sides of adjacent teeth on the same side of said central-conducting member having a geometric-sequence ratio 1', the ratio of the radial distance from the apex of the inner to outer sides of a given tooth being given by a geometric-sequence ratio 6, unbalanced transmission line means having opposite sides connected between said apex and said ground-plane.

8. A periodic antenna as defined in claim 7 in which said teeth are trapezoidal in contour.

9. A periodic antenna as defined in claim 7 in which said teeth are formed by rod-like conductors arranged along their contour.

10. A periodic antenna as defined in claim 9 in which said central-conducting member comprises a rod-like conductor electrically connected to said teeth.

11. A periodic antenna as defined in claim 7 in which post-supports are positioned adjacent to corners of said antenna portion, insulators being connected between said 7 posts and the corners of said antenna portion, insulated members connected between said teeth along their outer edges to support said antenna portion in a suspension manner, with said angles on and 11/2 being made catenary by suspension support.

References Cited in the file of this patent UNITED STATES PATENTS D. 184,971 Vitanza Apr. 21, 1959 8 Cabot Apr. 7, 1908 Pickles Aug. 2, 1949 Masters Aug. 30, 1949 Masters Aug. 30, 1949 Masters June 6, 1950 OTHER REFERENCES Institute of Radio Engineers Transaction, Part I, Marcy, 1957, pages 114-118. 

